Nature and timing of the latest Wisconsin advance of the James River lobe, South Dakota

Glaciation of the James River lobe area in eastern South Dakota (SD) produced several prominent geomorphic features of the Laurentide ice sheet record. During late Wisconsin time, the James lobe advanced southward in the James River lowland, overlapping the Prairie Coteau upland on the east and the Missouri Coteau on the west. The known southern limit of late Wisconsin till is the modern Missouri River Valley between Yankton and Richland, SD. North of the Missouri River, radiocarbon dates on wood in or beneath till from 12 sites in 10 counties of the southern James River lowland range from 12,050 to 12,880 14C yr B.P. and average 12,430 yr B.P. (~14,500 ka cal), with a mean reported uncertainty of 260 yrs. At most of these sites, including two within 20 km of Yankton, the dated wood is in till at depths as great as 58 m or in underlying sand and gravel. The dates are consistent with paleoecologic studies (e.g. Yansa, 2006) that indicate spruce parkland was in this region at that time prior to the glacial readvance. Outcrops of calcite-cemented glacial gravel cut by veins of banded travertine are present on a marginal escarpment of the James River valley, where it is incised into the 30-60-m-thick surface till of the James River lowland about 15 km north of the Missouri River. Twelve robust 230Th/U dates on calcite laminae ranging from 10.5 to 13.5 ka (mean 2s uncertainty of 0.4 ka) indicate that ground-water discharge from glacial-bed aquifers was closely related to the latest Wisconsin advance of the James lobe. Four dates on younger vug-filling calcite between 8.1 and 5.8 ka, as well as modern spring discharge, show decreasing initial 234U/238U with age. Stable-isotope data on dated travertine and modern spring water provide other means for evaluating environments of carbonate precipitation, including subglacial and/or subaerial settings and mixing of surface and ground water associated with ice-sheet dynamics. The 14C dates span the Bolling-Allerod time interval that ice core records show was markedly warmer than preceding glacial conditions. Thus, the late Wisconsin readvance of the James lobe, and similar readvances of the Des Moines and Lake Michigan lobes, may be complex dynamic responses of the Laurentide ice sheet to warming and increased meltwater.